Grease composition

ABSTRACT

Provided herein is a grease composition capable of reducing an amount of wear in sliding parts of various members of automobiles, electrical devices, and the like. The grease composition includes a lubricant base oil, a thickener, an amide compound, and at least one of an α-hydroxycarboxylic acid metal salt, and an ω-hydroxycarboxylic acid metal salt. The hydroxycarboxylic acid metal salts are contained in a total content of 0.1 to 2 mass % with respect to the total amount of the grease composition.

TECHNICAL FIELD

The present invention relates to a grease composition capable of reducing an amount of wear in sliding parts.

BACKGROUND ART

Grease has been mainly used for slide bearings and rolling bearings (bearings), or for sliding surfaces where a lubricant film cannot easily remain attached because of moving contact surfaces. Typically, grease is prepared by mixing a thickener, and, as required, additives, to a lubricant base oil.

With the advancement in machine technology such as in automobiles and electrical devices, these devices are used in increasing severe operating conditions the trend or downsizing and lighting, higher output, and longer life continues to grow. This has increased the demand for higher performance, for example, lubricity, in greases used for these devices.

Various options are proposed for the base oil, thickener, and additives to improve grease lubricity. For example, it has been proposed to add an amide compound and a solid lubricant to improve grease lubricity, for example, such as abrasion resistance (see PTL 1, and PTL 2). However, a grease having better lubricity is needed.

Hydroxyfatty acid metal salts are used as a thickener for grease (see, for example, PTL 3). However, the hydroxyfatty acid metal salts used as a thickener are mostly lithium 12-hydroxystearate, and an α- or ω-hydroxyfatty acid metal salt is not used. It is also known that use of this type of compound as an grease additive results in a shorter life, though it improves antirust performance (for example, PTL 4).

CITATION LIST Patent Literature PTL 1: JP-A-2013-181154 PTL 2: JP-A-2013-181156 PTL 3: WO2012/141222 PTL 4: JP-B-7-000796 SUMMARY OF INVENTION Technical Problem

The present invention was made under these circumstances, and it is an object of the present invention to provide a grease composition that is capable of reducing an amount of wear in sliding parts of various members of automobiles, electrical devices, and the like.

Solution to Problem

The present inventors diligently worked to achieve the foregoing object, and found that an amount of wear can be greatly reduced by the synergy between metal salts of specific hydroxycarboxylic acids and an amide compound. The present invention was completed on the basis of this finding, and includes the following.

(1) A grease composition comprising:

a lubricant base oil;

a thickener;

an amide compound; and

at least one of an α-hydroxycarboxylic acid metal salt, and an ω-hydroxycarboxylic acid metal salt,

wherein the hydroxycarboxylic acid metal salts are contained in a total content of 0.1 to 2 mass % with respect to a total amount of the grease composition.

(2) The grease composition according to item (1), wherein the hydroxycarboxylic acid metal salt is an alkali metal salt, or an alkali earth metal salt.

(3) The grease composition according to item (2), wherein the hydroxycarboxylic acid metal salt is a calcium salt, or a barium salt.

(4) The grease composition according to any one of items (1) to (3), wherein the hydroxycarboxylic acid is a monovalent hydroxycarboxylic acid of 8 to 24 carbon atoms.

(5) The grease composition according to any one of items (1) to (4), wherein the thickener contains at least one of a urea-based thickener, and a metal soap-based thickener.

(6) The grease composition according to any one of items (1) to (5), which further comprises a solid lubricant component.

(7) A lubrication method comprising using the grease composition of any one of items (1) to (6) at a sliding part.

Advantageous Effects of Invention

The grease composition of the present s highly effective at greatly reducing wear in sliding parts of various members of automobiles, electrical devices, and the like.

DESCRIPTION OF EMBODIMENTS Lubricant Base Oil

A lubricant base oil of the present invention may be a mineral or a synthetic lubricant base oil, as long as it is a lubricant base oil commonly used for grease. The lubricant base oil has a kinetic viscosity at 40° C. of preferably 1 to 500 mm²/s, more preferably 5 to 100 mm²/s. A grease composition having desirable consistency can be prepared with ease when the kinetic viscosity at 40° C. is 1 to 500 mm²/s. In order to prepare a grease having excellent lubricity, it is preferable to use a lubricant base oil of physical properties with a viscosity index of 90 or more, particularly 95 to 250, a pour point of −10° C. or less, particularly −15 to −70° C., and a flash point of 150° C. or more.

When containing a solid lubricant, the lubricant base oil preferably has a density of 0.75 to 0.95 g/cm³ at a lubricant base oil temperature of 15° C. The density is particularly preferably 0.8 to 0.9 g/cm³.

The mineral lubricant base oil may be a lubricant oil fraction obtained through purification of a nil by various purification processes after distillation of a crude oil under ordinary pressure or under ordinary pressure and reduced pressure. The purification processes include, for example, hydrorefining, solvent extraction, solvent dewaxing, hydrodewaxing, sulfuric acid washing, and clay treatment. The base oil of the present invention can be obtained from a combination of these processes performed in appropriate order. A mixture of different refined oils of different qualities obtained by treating different crude oils or different distillate oils with a combination of different processes in different order is also useful. Regardless of the method, the base oil can be desirably used when the base oil quality is adjusted to satisfy the foregoing density.

The synthetic lubricant base oil uses preferably a base material having desirable hydrolytic stability. Examples of such base materials include polyolefins such as poly-α-olefins, polybutene, and a copolymer of two or more olefins; polyesters, polyalkylene glycols, alkylbenzenes, and alkylnaphthalenes. Poly-α-olefins are preferred for availability, cost, viscosity characteristics, oxidation stability, and compatibility with system components. Further preferred for cost are poly-α-olefins made by polymerizing olefins such as 1-dodecene, and 1-decene.

Also preferred as the synthetic lubricant base oil are ester-based base oils, for example, such as a polyolester of a polyalcohol and a monovalent carboxylic acid, and a diester of a divalent carboxylic acid and an alcohol. Preferred are polyolesters obtained from a polyalcohol selected from neopentyl glycol, trimethylolpropane, and pentaerythritol, and a fatty acid of 4 to 22 carbon atoms. The fatty acid is particularly preferably a fatty acid of 12 to 20 carbon atoms, particularly an unsaturated fatty acid.

The lubricant base oil may use base oils other than the poly-α-olefinic base oil and the ester-based base oil. It is, however, preferable to contain 80 mass % or more, particularly 90 mass % or more of the poly-α-olefin or ester-based base oil with respect to the mass of the lubricant base oil.

The synthetic lubricant base oil may be used alone or as a mixture of two or more. It is also possible to use the synthetic lubricant base oil as a mixture with the mineral base oil.

The content of the lubricant base oil is preferably 50 to 95 mass %, particularly preferably 60 to 85 mass % with respect to the total amount of the grease composition. A grease composition having desirable consistency can be prepared with ease when the lubricant base oil content is 50 to 95 mass %.

Thickener

A thickener commonly used for grease may be used as the thickener of the present invention without causing trouble. It is, however, preferable to use a metal soap-based thickener, or a urea-based thickener. The thickener may be used alone or as a mixture of two or more. The thickener may be contained in any amount, as long as the desired consistency is obtained. For example, the thickener content is preferably 2 to 30 mass %, further preferably 5 to 20 mass % with respect to the total amount of the grease composition.

The metal soap-based thickener is a thickener of a carboxylic acid metal salt. The carboxylic acid may be a derivative of carboxylic acids having a hydroxy group.

The carboxylic acids may be aliphatic carboxylic acids such as stearic acid, and azelaic acid, or aromatic carboxylic acids such as terephthalic acid. Monovalent or divalent aliphatic carboxylic acids, particularly aliphatic carboxylic acids of 6 to 20 carbon atoms are used. Particularly preferred for use are monovalent aliphatic carboxylic acids of 12 to 20 carbon atoms, and divalent aliphatic carboxylic acids of 6 to 14 carbon atoms. Monovalent aliphatic carboxylic acids having one hydroxyl group are preferred.

The metal may be an alkali metal such as lithium and sodium, an alkali earth metal such as calcium, or an amphoteric metal such as aluminum. Preferred are alkali metals, particularly lithium.

Metal salts of α- or ω-hydroxyfatty acids are not preferred as thickener because of poor thickening capability.

The thickener may be added in the form of a metal soap, or as a metal-soap thickener by separately adding the carboxylic acid and a metal source (e.g., a metal salt, a metal salt hydroxide), and causing these to react at the time of grease production.

The carboxylic acid metal salt may be used alone or as a mixture of two or more. For example, a mixture of lithium 12-hydroxystearate and lithium azelate is particularly preferred.

The urea-based thickener may be, for example, a diurea compound obtained by a reaction of a diisocyanate and a monoamine, or a polyurea compound obtained by a reaction of a diisocyanate with a monoamine or a diamine.

Diisocyanates are compounds in which two of the hydrogen atoms of the hydrocarbon are substituted with isocyanate groups. Preferred examples of the diisocyanates include phenylene diisocyanate, tolylene diisocyanate, diphenyl diisocyanate, diphenylmethane diisocyanate, octadecane diisocyanate, decane diisocyanate, and hexane diisocyanate. The hydrocarbon may be a non-cyclic hydrocarbon or a cyclic hydrocarbon, or an aromatic hydrocarbon, an alicyclic hydrocarbon, or an aliphatic hydrocarbon. Preferably, the hydrocarbon is an aromatic hydrocarbon. The hydrocarbon has preferably 4 to 20 carbon atoms, particularly 8 to 18 carbon atoms.

Monoamines are compounds in which one of the hydrogen atoms of the ammonia is substituted with a hydrocarbon group. Preferred examples of the monoamines include octylamine, dodecylamine, hexadecylamine, stearylamine, oleylamine, aniline, p-toluidine, and cyclohexylamine. Diamines are compounds in which two of the hydrogen atoms of the ammonia are substituted with hydrocarbon groups. Preferred examples of the diamines include ethylenediamine, propanediamine, butanediamine, hexanediamine, octanediamine, phenylenediamine, tolylenediamine, xylenediamine, and diaminodiphenylmethane. The hydrocarbon groups of the monoamines and diamines may be non-cyclic hydrocarbon groups or cyclic hydrocarbon groups, or aromatic hydrocarbon groups, alicyclic hydrocarbon groups, or aliphatic hydrocarbon groups. Preferably, the monoamines and diamines contain aliphatic hydrocarbon groups. The monoamines and diamines have preferably 2 to 20 carbon atoms, particularly preferably 4 to 18 carbon atoms.

Hydroxycarboxylic Acid Metal Salt

The present invention contains at least one of an α-hydroxycarboxylic acid metal salt, and an ω-hydroxycarboxylic acid metal salt. It is preferable to contain both an α-hydroxycarboxylic acid metal salt, and an ω-hydroxycarboxylic acid metal salt, though only one of these may be contained. The metallic element forming the salt is preferably an alkali metal or an alkali earth metal, particularly preferably calcium or barium. The total content of the hydroxycarboxylic acid metal salts is 0.1 to 2 mass %, preferably 0.2 to 1 mass % with respect to the total amount of the grease composition. The wear reducing effect will not be obtained when the total content of the hydroxycarboxylic acid metal salts is less than 0.1 mass %, whereas the hydroxycarboxylic acid metal salt may inhibit the effects of other components when the content is above 2 mass %.

In the present invention, the α- and ω-hydroxycarboxylic acids are preferably monovalent, more preferably monovalent acids of 8 to 24 carbon atoms. The monovalent α-hydroxycarboxylic acid is represented by R¹—CH(—OH)—COOH, and the ω-hydroxycarboxylic acid is represented by HO—CH₂—R²—COOH. In the present invention, R¹ is preferably an alkyl group of 1 to 38 carbon atoms, or an alkenyl group of 2 to 38 carbon atoms, more preferably an alkyl group of 6 to 22 carbon atoms, or an alkenyl group of 6 to 22 carbon atoms. R² is preferably an alkylene group of 1 to 38 carbon atoms, or an alkenylene group of 2 to 38 carbon atoms, more preferably an alkylene group of 6 to 22 carbon atoms, or an alkenylene group of 6 to 22 carbon atoms.

Preferably, the present invention further contains metal salts of hydroxycarboxylic acids other than α- and ω-hydroxycarboxylic acids, or metal salts of carboxylic acids containing no hydroxyl group. Preferred examples of such carboxylic acid metal salts include alkali metal salts or alkali earth metals, preferably calcium or barium salts of α- and ω-hydroxycarboxylic acids in which the hydroxyl group is substituted at different positions from the foregoing α- and ω-hydroxycarboxylic acids, or in which two or more hydroxyl groups are substituted, and alkali metal salts or alkali earth metals, preferably calcium or barium salts of carboxylic acids that do not have a substitution of the hydroxyl group.

The hydroxycarboxylic acids, and the carboxylic acids may be obtained by chemical synthesis. It is, however, preferable to use a lanolin fatty acid—an acid obtained through purification (e.g., hydrolysis) of a wax-like substance adhering to sheep hairs—as a raw material these acids. When using a lanolin fatty acid as a raw material, the α- and ω-hydroxycarboxylic acids, and other preferred carboxylic acids may be separately used. It is, however, preferable to prepare the lanolin fatty acid directly into a metal salt, without separating these acids.

Amide Compound

The present invention further contains an amide compound. The amide compound, which is a compound having at least one amide group (—NH—CO—), may be a compound containing one amide group (monoamide), or a compound containing two amide groups (bisamide). Preferably, saturated aliphatic monoamides, or saturated aliphatic bisamides are used.

The saturated aliphatic bisamides are most preferred for their advantages including desirable heat resistance, and the ability to reduce the frictional resistance at a sliding part even with a relatively small amount.

The saturated aliphatic monoamides are amide compounds of a saturated aliphatic monoamine and a saturated aliphatic monocarboxylic acid. The saturated aliphatic bisamides may be amide compounds of a saturated aliphatic diamine and a saturated aliphatic monocarboxylic acid, or amide compounds of a saturated aliphatic dicarboxylic acid and a saturated aliphatic monoamine.

A preferred saturated aliphatic amide compound has a melting point of 100 to 170° C., and a molecular weight of 298 to 876.

The saturated aliphatic monoamides are represented by the following general formula (1). The saturated aliphatic bisamides are represented by the following general formulae (2) and (3).

R¹¹—CO—NH—R¹²  (1)

R¹³—CO—NH-A¹-NH—CO—R¹⁴  (2)

R¹⁵—NH—CO-A²-CO—NH—R¹⁶  (3)

In the formulae, R¹¹, R¹², R¹³, R¹⁴, R¹⁵, and R¹⁶ each independently represent a saturated aliphatic hydrocarbon group of 5 to 25 carbon atoms. In general formula (1), R¹² may be hydrogen. A¹ and A² represent divalent saturated aliphatic hydrocarbon groups of 1 to 10 carbon atoms, and are preferably divalent saturated chain hydrocarbon groups of 1 to 4 carbon atoms.

Specifically, preferred examples of the saturated aliphatic monoamides include lauramide, palmitamide, stearamide, and behenamide.

Preferred examples of the saturated aliphatic bisamides represented by formula (2) include ethylene bis(stearamide), ethylene bis(isostearamide), ethylene bis(lauramide), and methylene bis(lauramide). Preferred examples of the saturated aliphatic bisamides represented by formula (3) include N,N′-bisstearylsebacamide.

The bisamides are preferably amide compounds in which R¹³, R¹⁴, R¹⁵, and R¹⁶ in formulae (2) and (3) are each independently a saturated chain hydrocarbon group of 12 to 20 carbon atoms.

The amide compounds may be used alone or in a combination of two or more in any proportions. The content of the amide compound is preferably 1 to 30 mass %, more preferably 5 to 20 mass % with respect to the total amount of the grease composition.

It is to be noted here that the amide compound, when thermofused in the presence of the lubricant base oil, produces a state in which the lubricant base oil is held in the amide compound forming a three-dimensional network structure. This further lowers the coefficient of friction at a sliding part than when the amide compound is simply dispersed or mixed in the grease.

Solid Lubricant Component

Preferably, the present invention further contains a solid lubricant component.

The solid lubricant component may be, for example, melamine cyanurate, molybdenum disulfide, boron nitride, graphite, mica, fluororesin, fluorinated graphite, or a molybdenum organic compound commonly used for lubricants. The content of the solid lubricant component is preferably 0.1 to 10 mass %, more preferably 0.2 to 5 mass % with respect to the total amount of the grease composition.

In the present invention, it is more preferable to use boron nitride or melamine cyanurate as the solid lubricant component. Particularly preferably, a hexagonal, atmospheric-phase boron nitride (h-BN) powder of a particle size suited for the intended purpose may be appropriately selected and used. The solid lubricant component has a particle diameter of preferably 1 to 10 μm.

Other Additives

Additives commonly used for lubricant oils and greases may be appropriately added, as required, to the grease composition of the present invention, in addition to the foregoing components. Examples of such additives include cleaners, dispersants, antiwear agents, viscosity index improvers, antioxidants, extreme-pressure agents, anti-rusting agents, and corrosion inhibitors.

Subject of Lubrication

The grease composition of the present invention is preferably used at sliding parts, preferably for lubrication of various sliding members, for example, between metal parts, metal and resin parts, and resin parts.

The sliding member resin may be a natural resin or a synthetic resin. Preferred as synthetic resins are general-purpose plastics (e.g., polyethylene, polystyrene, polypropylene, and polyvinyl chloride), and engineering plastics. Engineering plastics are particularly preferred in terms of heat resistance, and mechanical strength. Examples of synthetic resins include polyamide resins, polyacetal resins, polycarbonate resins, polysulfone resins, polyphenylene sulfide resins, polyamideimide resins, polyetheretherketone resins, phenolic resins, polyester resins, and epoxy resins. The grease composition is particularly preferred for lubrication of polyamide resins, and polyoxymethylene resins.

Examples of the applicable areas of the grease composition of the present invention include transport machinery such as automobiles, railway, and aircraft; industrial machines such as machine tools; home electrical appliances such as washing machines, refrigerators, and vacuum cleaners; and precision machines such as watches and cameras. The grease composition has use for, for example, bearings, gears, sliding surfaces, belts, joints, and cams used for these machines. The grease composition is particularly useful for improving the abrasion resistance of gears (e.g., spur wheels, helical gears, crossed helical gears, hypoid gears, worm gears, and wheel gears) exposed to a high-surface-pressure sliding environment.

EXAMPLES

The present invention is described below using Examples representing implementations of the present invention. The present invention, however, is not limited to the following implementations.

Preparation Method

Each component was put in a container in the amounts (mass %) shown in Tables 1 and 2, and heated to 150° C. (a temperature equal to or greater than the melting point of the amide compound). The mixture was then stirred with a magnetic stirrer, and cooled to room temperature. The product was subjected to a dispersion process under applied pressure using rollers (three rolls) to prepare a grease composition.

The components used are as follows.

1. Lubricant Base Oil:

(1) Poly-α-Olefin (Durasyn 170 Available from INEOS)

Kinetic viscosity at 40° C.: 68 mm²/s

Density at 15° C.: 0.83 g/cm³

Viscosity index: 133

Pour point: −45° C.

Flash point: 250° C.

(2) Polyolester (an Ester of Trimethylolpropane and an Oleic Acid of 18 Carbon Atoms)

Kinetic viscosity at 40° C.: 46 mm²/s

Density at 15° C.: 0.92 g/cm³

Viscosity index: 146

Pour point: −32.5° C.

Flash point: 310° C.

2. Thickener: (1) Urea-Based Thickener

Alicyclic diurea of cyclohexylamine and methylene diphenyl diisocyanate

(2) Lithium-Based Thickener

A lithium salt of a mixed acid of 12-hydroxystearic acid and azelaic acid

3. Solid Lubricant Component: (1) Boron Nitride

Hexagonal boron nitride having an average particle size of 2 μm as measured by laser diffractometry

(2) Melamine Cyanurate (MCA)

Melamine cyanurate having an average particle size of 1 μm as measured by laser diffractometry

4. Amide Compound: (1) Ethylene Bis(Stearamide) (Guaranteed Reagent) (2) Ethylene Bis(Lauramide) (Guaranteed Reagent) 5. Carboxylic Acid Metal Salt (1) Barium Lanolate

Content of α-hydroxycarboxylate: 30 mass %

Content of ω-hydroxycarboxylate: 5 mass %

Content of hydroxycarboxylates other than α- and ω-hydroxycarboxylates: 1 mass %

Content of carboxylate having no hydroxyl group: 40 mass %

Number of carbon atoms in hydroxycarboxylic acid: 8 to 24

Number of carbon atoms in carboxylic acid having no hydroxyl group: 8 to 24

(2) Calcium Lanolate

Content of α-hydroxycarboxylate: 30 mass %

Content of ω-hydroxycarboxylate: 5 mass %

Content of hydroxycarboxylates other than α- and ω-hydroxycarboxylates: 1 mass %

Content of carboxylate having no hydroxyl group: 40 mass %

Number of carbon atoms in hydroxycarboxylic acid: 8 to 24

Number of carbon atoms in carboxylic acid having no hydroxyl group: 8 to 24

Evaluation of Amount of Wear

Evaluation was made according to the ASTM D 3233 test method for measurement of load bearing properties of lubricant oil, using a Falex friction tester. In this test, a resin (polyamide resin (PA66)) pin was held between two steel (SUJ-2) V blocks, and a sample was charged into a sample container. A 55-MPa load was applied with the rotational axis held stationary, and the lubricant oil was tested for load bearing properties at an oil temperature of 120° C. under a rotation speed of 360 rpm. The amount of wear was measured by measuring the reduction of the pin weight after 10 minutes. The results are presented in Tables 1 and 2.

Evaluation Result

The amount of wear was considerably smaller in the grease compositions of Examples that contained at least one of the α-hydroxycarboxylic acid metal salt and the co-hydroxycarboxylic acid metal salt with the thickener and the amide compound. On the other hand, there was no large reduction in the amount of wear when the hydroxycarboxylic acid metal salts were not contained, even when simple carboxylates, or a phosphite ester (a known antiwear agent) were added (Comparative Examples 1 to 7).

TABLE 1 Compar- Compar- Compar- Compar- Compar- Compar- Exam- Exam- Exam- Exam- ative ative ative ative ative ative ple 1 ple 2 ple 3 ple 4 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Lubricant base oil Poly-α-olefin Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Thickener Urea-based 10 10 10 10 10 10 10 10 10 10 thickener Amide compound Ethylene bis- 15 15 15 15 15 15 15 15 15 15 stearylamide Solid lubricants Boron nitride 1.0 1.0 1.0 — 1.0 1.0 1.0 1.0 1.0 1.0 Carboxylic acid Barium lanolate 1.0 2.0 — 2.0 — — — — — — metal salt Calcium lanolate — — 1.0 — — — — — — — Content of α-hydroxycarboxylic acid 0.35 0.70 0.35 0.70 — — — — — — metal salt and ω-hydroxycarboxylic acid metal salt Other additive Oleylamide — — — — — 1.0 — — — — compounds Aluminum stearate — — — — — — 1.0 — — — Zinc naphthenate — — — — — — — 1.0 — — Phosphite ester — — — — — — — — 1.0 — Phosphate — — — — — — — — — 1.0 Evaluation results Worked penetration 324 308 321 330 315 293 306 316 330 328 Amount of wear 0.5 0.6 0.8 0.8 1.3 0.9 1.0 1.0 0.9 1.2 (mg)

TABLE 2 Comparative Example 5 Example 6 Example 7 Example 8 Example 9 Example 10 Example 7 Lubricant base oil Poly-α-olefin Balance Balance Balance Balance — Balance Balance Polyolester — — — — Balance — — Thickener Litium-based 10 10 10 10 10 10 10 thickener Amide compound Ethylene bis- 15 15 — 15 15 15 — stearylamide Ethylene bis- — — 15 — — — — laurylamide Solid lubricants Boron nitride — 2.0 2.0 — 2.0 2.0 — MCA — — — 2.0 — — — Carboxylic acid metal Barium lanolate 1.0 1.0 1.0 1.0 1.0 — — salt Calcium lanolate — — — — — 1.0 — Content of α-hydroxycarboxylic acid 0.35 0.35 0.35 0.35 0.35 0.35 — metal salt and ω-hydroxycarboxylic acid metal salt Evaluation results Worked penetration 320 280 300 274 298 280 300 Amount of wear 1.2 0.8 1.0 1.0 1.0 1.0 1.6 (mg)

INDUSTRIAL APPLICABILITY

The grease composition of the present invention can greatly reduce an amount of wear in sliding members, and is useful for lubrication of sliding parts of various members of automobiles, electrical devices, and the like. 

1. A grease composition comprising: a lubricant base oil; a thickener; an amide compound; and at least one of an α-hydroxycarboxylic acid metal salt, and an ω-hydroxycarboxylic acid metal salt, wherein the hydroxycarboxylic acid metal salts are contained in a total content of 0.1 to 2 mass % with respect to a total amount of the grease composition.
 2. The grease composition according to claim 1, wherein the hydroxycarboxylic acid metal salt is an alkali metal salt, or an alkali earth metal salt.
 3. The grease composition according to claim 2, wherein the hydroxycarboxylic acid metal salt is a calcium salt, or a barium salt.
 4. The grease composition according to claim 1, wherein the hydroxycarboxylic acid is a monovalent hydroxycarboxylic acid of 8 to 24 carbon atoms.
 5. The grease composition according to claim 1, wherein the thickener contains at least one of a urea-based thickener, and a metal soap-based thickener.
 6. The grease composition according to claim 1, which further comprises a solid lubricant component.
 7. A lubrication method comprising using the grease composition of claim 1 at a sliding part. 